1. Field of the Invention
This invention relates to a self incinerating oven and process carried out thereby and more particularly to an apparatus and method by which the volatile organic compounds generated during the curing process of various coatings, can be converted to harmless products of combustion within the structure of a radiant oven, while, at the same time, utilizing the energy contained in the discharged gases from the incineration process to heat the radiation emitting wall of the oven.
2. Description of the Prior Art
Volatile organic compounds (VOC's) are generated in the curing processes of many coatings on surfaces, such as on the surfaces of vehicle bodies.
In most industrial applications, the oven exhaust, containing these VOC's, is collected and ducted to a common incinerator. Usually, one central incinerator is used to incinerate the exhaust from multiple ovens. Sometimes as many as ten ovens are involved. There are many disadvantages to the central type of incinerator that is used to incinerate the exhaust from multiple processes.
The most common problem associated with a central incinerator system is the complex duct work required to accumulate the exhaust from multiple ovens and to transport it to a central incinerator. Because of the excess energy generated from the incineration process, additional high temperature ducts are then required to distribute the high temperature gases back into processes which can utilize the energy. In most industrial application, where the source of the VOC's are from oven exhaust, the excess energy from the incinerator is used to provide a portion or all of the heat for the ovens. Because of the high temperature of the gases discharged from the incinerator, in many industrial applications, heavily insulated, seam welded duct work, for distributing the high temperature gases are used.
If the high temperature gases are to be used to heat ovens, usually different oven zones will require different amounts of energy. Therefore, it has become necessary to control the volume of high temperature air to each oven zone through high temperature dampers and control systems. Leakage is a common problem with high temperature dampers and, in general, the construction of the high temperature ducts is more complex and costly than is required for lower temperature air (under 500.degree. F.).
Central incinerators require valuable industrial floor space and the complex duct distribution system associated with a central incinerator require valuable space, and certainly clutters the area.
Another common problem with a central incinerator on which multiple processes are dependent, is that a major portion of a finishing line can be disrupted due to a central incinerator failure.
The concentration of VOC's contained in the air expelled from ovens used for drying or curing various types of coatings, is usually much below a concentration required for producing a combustible mixture or even for the lower explosion limit (LEL).
Therefore, these VOC's have to be oxidized to harmless gases by incineration. They cannot be converted to products of combustion simply by being burned because of their low concentration in the mixture. However, when these compounds are exposed to an increased temperature for a sufficient time in the presence of oxygen, they can be oxidized into harmless products of combustion. This process is usually accomplished at temperatures of between 1250.degree. F. and 1500.degree. F. and with dwell times for the organic matter from 0.2 seconds to 1 second.
With ovens, where incineration of the VOC's are required, the temperature of the mixture containing the VOC's is relatively low, 250.degree. F. to 350.degree. F., compared to the temperature required for oxidation of the VOC's. Therefore, in order to conserve energy, most incinerators that are remote from the oven are of the recuperative type which allow for pre-heating the incoming mixture containing VOC's by the hot gases generated from the incineration process. Therefore, the energy added to accomplish the incineration can be greatly reduced if the incoming mixture can be separated from the combustion gases and pre-heated normally to 800.degree. F.-1000.degree. F. If pre-heating can be accomplished to this temperature, then the energy added to the mixture to be incinerated is only that required to heat the mixture 800.degree. F.-1000.degree. F. to the incineration temperature, which is usually in the range of 1250.degree. F. to 1500.degree. F.
In order to pre-heat the incoming mixture containing the VOC's, some type of air-to-air heat exchanger is required. A typical heat exchanger used for this application would employ tubular heat transfer surfaces which could be constructed in many geometries. Most practical heat exchangers of this type use either cylindrical or rectangular tubular heat transfer surfaces.
In conventional incinerators employing pre-heating, it would not be desirable to mix the incoming polluted air mixture with the relatively clean air from the incineration process. Therefore, the heat exchanger has required gas tight seals to prevent the intermixing of the two. This gas tight seal is usually produced by welding the heat transfer tubular surfaces into headers. In many designs, the tube headers, in which the heat transfer tubes are welded, are exposed to the high exhaust gas temperatures of the incinerator. Failure of these welds and the heat exchangers associated with these types of incinerators have been a common problem in the past. Also, the failure of these heat exchangers has created expensive repairs.
A problem associated with the type of heat exchangers described above is that the resulting structure must provide for the expansion and contraction of the multiple tubes contained within one heat exchanger when different tubes are exposed to different temperatures. Carbide precipitation within the weld, is another problem in prior art devices and in many instances, the weldment is subjected to high stresses as a result of the expansion and contraction of the heat transfer tubes.
Attempts have been made, in the past, to only weld one end of each heat transfer tube into one of the tube plates and allow the other end to float in the other tube plate. While this concept does provide for expansion and contraction of the tubes, it adds another design problem of providing a means to seal each tube while it expands and contracts within the tube plate. Thus, these types of incinerators have not been satisfactory or widely accepted because of leakage at the seal.
The present invention overcomes the difficulties prescribed above by providing an inexpensive and yet quite durable and efficient oven containing an incinerator or combustion chamber which achieves virtually 100% destruction of VOC's. I have also found that it is not necessary to provide a specific dwell time for the polluted air in the combustion chamber at a specific constant elevated temperature. In other words, the mixture of incoming air and VOC's does not have to be held at a specified constant temperature before it enters the primary heat exchanger. The rate at which the VOC's are oxidized depends upon, among other things, the temperature of the mixture. Thus, the dwell time required will depend upon the temperature versus time relationship experienced by the mixture as it passes through the combustion chamber.
In the preferred embodiment of this invention, air is circulated from an oven into a primary heat exchanger contained within the confines of an incineration chamber and is preheated before it is discharged from the heat exchanger tube into the incinerator chamber. The VOC's are then converted to products of combustion in the presence of oxygen at a sufficient temperature and time for this oxidation to occur. The hot gases from the incineration process is then used to provide the primary heat energy to a radiant wall for transferring heat to objects passing through the oven. This concept provides for an efficient method for the destruction of essentially 100% of the VOC's while at the same time insuring sufficient heat energy for the oven.